As of change 891802 (T330228), patches to update mediawiki/vendor are consistently failing with:
Error: your composer.lock file is not up to date.
Interestingly, it is not reporting which packages are missing or outdated. This appears to be an impossible condition because maintenance/checkComposerLockUpToDate.php (as called from composer phpunit bootstrap), explicilty checks whether the two "problem" arrays are empty.
In addition to not printing out what the problem is, it also appears unfixable. @Reedy and myself have both tried to generate the lock file cleanly and it won't accept it. This is at https://gerrit.wikimedia.org/r/c/mediawiki/vendor/+/896656/ (for wikimedia/less.php) and https://gerrit.wikimedia.org/r/c/mediawiki/vendor/+/901744/ (wikimedia/minify).